With the revolution in semiconductor self-commutating devices and reduction in their cost, most of the electric utilities in industrial, commercial, residential, aerospace environments make use of some kind of power processors. These power processors are being used in applications such as Heating, Ventilation and Air Conditioning (HVAC) systems, pumps, blowers, fans, paper and textile mills, rolling mills and the like. These power processors generally use non-linear devices such as diode, thyristor, IGBT etc., which results in injection of current harmonics, resulting in equipment overheating, low rectifier efficiency, malfunction of sensitive electronic equipments and the like. These harmonic currents result in voltage distortion (as they travel through finite source impedance) at Point of Common Coupling (PCC), thereby affecting the nearby consumers. To limit these harmonics, an IEEE Standard entitled “IEEE Guide for harmonic control and reactive compensation of Static Power Converters, IEEE Standard 519-1992” was reissued in 1992.
Different techniques based on multipulse converters have been proposed to shape ac input current waveforms in phase with input voltage. The conventional wye-delta transformer based 12-pulse rectification scheme is one such example. But it is very difficult to build wye and delta connected windings with comparable electrical characteristics (voltage and impedance). Moreover, the kVA rating of the transformer is higher than the active power drawn by the converter. To reduce the transformer rating, autotransformer based multipulse converters are used. In the autotransformer, the windings are interconnected such that the kVA rating of the magnetic coupling is only a fraction of the total kVA of the drive, resulting in reduction in size and weight of the transformer.
For applications where the demand for harmonic current reduction is more stringent, the 18-pulse AC-DC converter is generally preferred. These converters are more economical than the 24-pulse AC-DC converter, while being more effective than the 12-pulse AC-DC converter. Autotransformer based 18-pulse AC-DC converter has been described in U.S. Pat. No. 4,876,634 entitled “Multipulse converter system” to reduce the Total Harmonic Distortion (THD) of ac mains current. This results in higher dc-link voltage by about 14%, making the scheme non applicable for retrofit applications. To overcome the problem of higher dc link voltage, different configurations of 18-pulse ac-dc converters have been reported in U.S. patents such as U.S. Pat. Nos. 5,619,407, 601,113 etc. These configurations use autotransformers having three coils having a plurality of serial windings and a plurality of stub windings. The stub windings are magnetically coupled with the serial windings from the same coil. The serial windings may form either a delta or a wye connection. Further, in these configurations, the voltages from stub windings are staggered at 40 degrees with respect to the supply voltages. These solutions result in a higher size winding and core material.
Thus, it would be advantageous to have an autotransformer for converting three-phase AC voltages to nine phase-shifted voltages with less number of windings resulting in reduced magnetics rating, simple design of the transformer and which is also suitable for retrofit applications and further which may perform satisfactorily even under load variation on the dc side.